Haven't seen a note on this, so might be helpful to those with pressurized systems and not connected to a water line. I have no water where my boiler is located, so I have to add water manually when needed (rarely, usually only when I want to work on the system). An easy way to de-pressurize the system to add water or otherwise work on the system is to have a shut-off valve on the pressure tank, and then when the system is hot, fire out, turn the valve off. The system pressure will fall to 0 or less as the system cools (might suck in some air) because the pressure tank will not release its stored water, you can work on the system, and additional water can easily be added to a high fill point. When finished adding water, close the fill valve, open the pressure tank valve, and the system is instantly re-pressurized, and any air admited is expelled through the vents.